code: def test_backtest_multiple_series(self):
      series = [
          AirPassengersDataset().load(),
          MonthlyMilkDataset().load(),
      ]
      model = NaiveSeasonal(K=1)
  
      error = model.backtest(
          series,
          train_length=30,
          forecast_horizon=2,
          stride=1,
          retrain=True,
          last_points_only=False,
          verbose=False,
      )
  
      expected = [11.63104, 6.09458]
      assert len(error) == 2
      assert round(abs(error[0] - expected[0]), 4) == 0
      assert round(abs(error[1] - expected[1]), 4) == 0
  
imports: import random
  from itertools import product
  import numpy
  import pandas
  import pytest
  from darts import TimeSeries
  from darts.datasets import AirPassengersDataset, MonthlyMilkDataset
  from darts.logging import get_logger
  from darts.metrics import mape, r2_score
  from darts.models import ARIMA, FFT, ExponentialSmoothing, NaiveDrift, NaiveSeasonal, Theta
  from darts.tests.conftest import tfm_kwargs
  from darts.utils.timeseries_generation import gaussian_timeseries
  from darts.utils.timeseries_generation import linear_timeseries
  from darts.utils.timeseries_generation import random_walk_timeseries
  from darts.utils.timeseries_generation import sine_timeseries
label_desc: Use image node_id 2 for calling the TestBacktesting obj's underlying member method code with example usage: obj.test_backtest_multiple_series() without return types